Dihydropyrimidine dehydrogenase (DPD) is the rate limiting enzyme in the metabolic pathway leading to the catabolism of pyrimidines. Catabolism of uracil also results in the synthesis of B-alanine. Thus, DPD is a critical enzyme in intermediary metabolism. In addition, DPD is responsible for catabolism of the chemotherapeutic drug, 5-fluorouracil. A deficiency in DPD was detected in patients who exhibited toxicity when administered 5-fluorouracil. These subjects had mean lymphocyte DPD activities that were one-half those of the average population. A complete lack of DPD activity has been associated with a variety of birth defects in affected individuals. To determine the mechanism of the DPD deficiency and to develop a diagnostic test for the defect, the human DPD cDNA gene was isolated and mutant DPD genes were characterized. A Dutch family having a child lacking DPD activity was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR). The DPD gene in this family was found to encode a defective RNA transcript that lacked an exon due to exon skipping during the pre-mRNA splicing. The affected child was homozygous for the defective DPD gene. The complete DPD gene was cloned in two overlapping mega YACS and is currently being sequenced. This will allow the direct sequencing of DPD genes from deficient individuals.